1. Field of the Invention
The present invention relates to integrated circuits and, in particular, relates to miniaturized electrical interconnects having reduced resistance and capacitance.
2. Description of the Related Art
To provide improved performance, manufacturers of integrated circuit devices continually strive to increase circuit density. Such devices are typically formed on a semiconductor substrate, such as a silicon wafer, and comprise a large number of miniaturized circuit elements. These elements, which include transistors, diodes, capacitors, and resistors, are usually disposed within or adjacent the substrate and define a plurality of circuit nodes. To combine the circuit elements into a useful electronic circuit, integrated circuit devices typically include a plurality of conducting paths that link the circuit nodes in a preferred manner. Typically, the conducting paths are provided by electrical interconnects comprising metallic wires including, for example, wires made of aluminum or aluminum alloy that are embedded in an insulating layer such as a layer of insulating SiO2.
However, as circuit density is increased, problems associated with conventional electrical interconnects are becoming more apparent. In particular, a higher density device having an increased number of circuit elements will likely require an even greater increase in the number of electrical interconnects. Consequently, the electrical interconnects will need to have a reduced thickness and adjacent interconnects will need to be spaced more closely together.
Unfortunately, such dimensional reductions tend to increase the resistance of individual interconnects and increase the capacitance between adjacent interconnects, thereby possibly increasing signal propagation delays and signal cross-talk. In particular, electrically charged adjacent conductors acts as the plates of the capacitors. As the distance between adjacent conductors decrease, the resulting capacitance increases. This resulting increase in capacitance slows propagation of signals as the capacitance must be overcome prior to propagation of the signal along the conductor. Hence, while it is desirable to increase device density on integrated circuits, considerations such as these pose problems for maintaining or improving circuit performance.
To improve the conductivity of interconnects, it has been suggested that copper metallurgy be substituted for the aluminum metallurgy that is now typically being used. Advantageously, copper metallurgy interconnects are viewed as having increased conductivity and thus less resistance. The lower resistance of interconnects of this metallurgy could allow the use of smaller dimensions of interconnects thereby facilitating the increase of device density on the integrated circuit. However, several potential problems have been encountered in the development of this proposed metallurgy. One of the main ones being the fast diffusion of copper through both silicon and SiO2. Fast diffusion of copper into silicon or silicon oxide results in diffusion of the conductive interconnect into the surrounding materials which can damage device performance or can result in short circuits occurring between adjacent interconnects.
To decrease capacitive loading, it has been suggested that the interconnects could be embedded in a solid insulating medium other than SiO2, such as a polymer comprising fluorinated polymide. However, as in the case of SiO2, an incompatibility problem with copper metallurgy has been found. In the case of polyimide, and many other polymers, it has been found that the polymer, during the curing, reacts with copper forming a conductive oxide CuO2, which is dispersed within the polymer. This then raises the effective dielectric constant of the polymer and in many cases increases its conductivity. Hence, there have been numerous suggested approaches towards solving the problems of capacitive coupling and increased resistance occurring as a result of a need to formulate smaller dimensioned interconnects that are spaced closer together. A primary difficulty that results is the relative incompatibility of lower resistance materials with the surrounding insulating material.
Silver is the ultimate conductor, in that it has the lowest specific resistivity of any metal or alloy. Furthermore, a vacuum is the ultimate dielectric, with air being nearly as good. However, the use of a vacuum introduces additional problems or complexities to the device. The first being the low heat conductivity of the vacuum and the second being the cost of the package required to maintain the vacuum. Air, which has somewhat better thermal conductivity, has its own problems in that both copper and silver react with air to form oxides or other compounds. Alternatively, gold is known to be quite environmentally stable. However it""s specific resistivity is higher than that of copper and silver.
To address the problem of increased capacitance, interconnects comprising an air bridge have been developed as described in U.S. Pat. No. 5,891,797. The air bridge is a length of conducting material that extends from a first supported end to a second supported end through an air space such that the bridge is substantially surrounded by air. Consequently, because air has a dielectric constant that is substantially less than that of SiO2, the capacitance between adjacent interconnects is reduced.
However, because the air bridge tends to sag under its own weight, the length of the air bridge is a possible limiting factor. In particular, because the air bridge is only supported at its first and second ends, gravitational forces acting on the air bridge when the bridge is horizontally disposed cause the air bridge to sag such that the unsupported middle of the air bridge is deflected downward with respect to the first and second ends. Because the degree of sagging increases as the length of the bridge is increased, the length of the air bridge cannot exceed that which would cause the air bridge to break or come into contact with another conductor of the device.
According to classical mechanics for simple air bridge structures, the center of the bridge is deflected downward with respect to the supported and constrained ends by an amount xcex4 given by       δ    =                  ρ        ⁢                  xe2x80x83                ⁢                  L          4                            32        ⁢                  xe2x80x83                ⁢                  h          2                ⁢        E              ,
wherein xcfx81 is the mass per unit volume of the air bridge, L is the length of the air bridge, h is the height of the air bridge, and E is the modulus of elasticity of the air bridge. Consequently, aside from the geometric factors L and h, the deflection xcex4 is proportional to the ratio of (xcfx81/E). Thus, an air bridge formed of a material having a reduced ratio of (xcfx81/E) will experience less sagging. If the ends of the bridge are not considered to be constrained then   δ  =                    5        ⁢                  xe2x80x83                ⁢        ρ        ⁢                  xe2x80x83                ⁢                  L          4                            32        ⁢                  xe2x80x83                ⁢                  h          2                ⁢        E              .  
This is the worst case assumption.
The table above illustrates the physical properties of possible air bridge materials. Both copper and silver have resistivities that are substantially less than that of aluminum and, thus, would provide air bridges with reduced resistance. Because copper has a ratio of (xcfx81/E) which is less than that of silver, a low resistance bridge comprising copper would experience less sagging and, thus, would be more suitable for applications that require bridges having extended lengths. Alternatively, because silver has a resistivity less than that of copper, a bridge comprising silver would be more suitable for applications that require reduced resistance. However, as was pointed out previously, both copper and silver are susceptible to environmental degradation in an air environment.
Gold also has a resistivity less than that of aluminum. Furthermore, gold is not susceptible to environmental degradation in an air environment. However, because the resistivity of gold and the ratio of (xcfx81/E) of gold are substantially larger those of silver or copper, a bridge formed of gold would have a relatively large resistance and would experience a relatively high degree of sagging.
From the foregoing, therefore, it will be appreciated that there is a need for an improved air bridge for an integrated circuit that not only provides a relatively small resistance but also is extendable over relatively large distances.
The aforementioned needs are satisfied by one aspect of the present invention which is an electrical interconnect of an integrated circuit for electrically connecting first and second nodes of the integrated circuit. The electrical interconnect comprises a bridge supported at a first and second end such that gravitational forces acting on the bridge cause the bridge to sag. The bridge is disposed adjacent a gaseous medium so as to reduce the capacitance of the interconnect. The bridge comprises a conductive core that includes a material which has a tendency to become contaminated by the gaseous medium. The bridge further comprises a protective coating covering a substantial portion of the core so as to inhibit the core from being contaminated by the gaseous medium.
In one embodiment, the conductive core comprises a material selected from the group comprising copper and silver. Because these materials have a resistivity less than that of conventional wiring materials, i.e. aluminum, the bridge is able to have a substantially reduced resistance. Furthermore, the protective coating can comprise either a conductive material or an insulating material.
In another aspect, the aforementioned needs are provided by an air bridge for electrically connecting first and second nodes of an integrated circuit. The air bridge comprises a core that provides a substantial portion of the air bridge. The core comprises a material selected to provide a reduced resistance and is disposed adjacent an air space comprising air so as to reduce the capacitance of the air bridge. The air bridge further comprises a protective coating substantially surrounding the core and interposed between the core and the air space. The protective coating inhibits air from the air space from diffusing through and contaminating the core.
In yet another aspect, the aforementioned needs are provided by an integrated circuit having a plurality of circuit nodes. The integrated circuit comprises a first interconnect that provides a first conducting path between first and second circuit nodes and comprises a first air bridge extending through an air space. The first air bridge comprises an inner core that includes a first material selected to provide the first air bridge with a resistance substantially less than that of aluminum and a mass density substantially less than that of gold. The first air bridge further comprises a protective coating that includes a second material selected to inhibit oxygen from diffusing therein.
The protective coating substantially surrounds the core and inhibits oxygen from the air space from contaminating the core. The integrated circuit further comprises a second interconnect providing a second conducting path between third and fourth circuit nodes. The second interconnect comprises a second air bridge which is substantially identical to the first air bridge and extends through the air space so that the second air bridge is disposed adjacent the first air bridge.
In still yet another aspect, a method of forming an electrical interconnect for electrically connecting first and second nodes of an integrated circuit is provided. The interconnect extends from a first supported end to a second supported end through a gaseous medium such that gravitational forces acting on an unsupported midsection of the electrical interconnect induce the unsupported midsection to sag with respect to the first and second supported ends of the interconnect. The method comprises forming a core section of the interconnect of a first conducting material selected to provide the core section with a reduced resistance. The method further comprises depositing a protective coating on a surface of the core section so as to inhibit the gaseous medium from reacting with the core section.
From the foregoing, it should be apparent that electrical interconnect of the present invention and methods of providing the same provide many advantages over interconnects known in the art. In particular, because the bridge section of the interconnect is disposed adjacent an air space instead of a solid insulating material, the bridge is provided a reduced capacitance. Furthermore, because the material of the core of the bridge is more conductive than that which is used in typical interconnects, the interconnect of the present invention can be formed with an increased length and a reduced cross-sectional area. Moreover, because the protective coating inhibits the air adjacent the core from contaminating the core, the core can comprise environmentally sensitive materials that provide improved conductive and mechanical properties. These and other objects and advantages of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.